This invention relates to an apparatus for performing optical loop back and line tests in an optical subscriber transmission system.
FIG. 1 is a schematic block diagram of the testing set-up specified by the CCITT (International Telegraph and Telephone Consultative Committee). This figure illustrates the basics of Loop 1 test, in which the signal transmitted from the network-side to the line terminal LT is returned back to the network-side at the user-side of the line terminal LT. In Loop 2 test, the signal transmitted from the network-side to the user-side is looped back at the user-side of the network terminal NT 1 to the network-side. These two tests are concerned with evaluating the network-side of the communication system. Loop C test is concerned with evaluating the user-side of the system, including the network terminal NT 1, by sending the signal sent from the terminal equipment TE1 to NT 1 back to TE1 at the network-side of the network terminal NT 1.
The two conventional methods of testing the optical subscriber transmission system will be explained with reference to FIG. 2.
The first test is concerned with evaluating the performance of the transmission cable 7 between LT and NT. The method of testing, as explained in detail in a report, a first reference, ("Fiber line Support System-AURORA & FITAS", N. Tomira, K. Sato and I. Nakamura, NTT Review, vol. 3, No.1, January, 1991), utilizes a test beam generated by an optical time domain reflectometer (OTDR) 10, which is passed through a optical directional coupler 3 into a transmission cable 7, and is reflected at a wavelength selection filter 8 (which selectively transmits the optical signal beam of a certain wavelength but reflects the test beam of testing wavelength) back into the OTDR 10. This test is for evaluating the cable 7 in the transmission Region II between the optical directional coupler 3 and the user/network division point. The optical directional coupler 3 (henceforth referred to as the directional coupler 3) is provided with a terminal T1 on the network-side, a terminal T2 on the user-side and a terminal T3 on the OTDR-side. The directional coupler 3 used in this test has a special characteristic of generating an asymmetric coupling ratio: a low value attenuation between T1--T2 and a high value of attenuation between T2--T3. To prevent adversely affecting the performance of the transmission cable 7 by the reflected test beam, reflected at the user/network division point and transmitting through the path T2--T1, the system is provided with a wavelength selection filter 8 (henceforth referred to as the optical filter 8) so as to reflect on the test beam back into OTDR 10.
The second group of tests is concerned with evaluating the performance of the system in Loop 1 and Loop 2. The signal transmitted from network-side is looped back at LT (Loop 1) and at NT (Loop 2). These test are all tests based on electrical looping back.
The conventional testing methodology is acceptable when the loop back point coincides with the user-network interface (T reference point). For example, with reference to FIG. 2, Loop 2 test is able to identify whether a problem exists in the network-side. That is, if no problem is found by Loop 2 test, the performance of the transmission line between LT 1 and T reference point is deemed to be problem-free. However, because there have been cases of NT 9 being owned by the users, there has been a Growing demand in recent years for Loop C test which can evaluate the performance of the user-side of the transmission system including any optical devices, and which can identify problems in the network-side separately from those in the user-side.
The problems with the conventional transmission line test (henceforth referred to as the line test) and the methodology for Loop 1 and Loop 2 tests are summarized in the following.
(1) The line test is only for region II, and it was not possible to test for the regions I and III, and it was time-consuming to identify faults in the system.
(2) Even if the OTDR is applied to testing of the region III, because the conventional tests are designed for measuring only the transmission loss and reflection attenuation, the optical transmission performance (such as optical pulse width, extinction ratios and noises in the receiver) in LT 1 could not be evaluated.
(3) The Loop 2 test is able to perform overall testing of the regions I, II and III, but because it is not able to distinguish these regions, it was not able to pinpoint the fault point.
(4) Because an optical testing device for performing Loop 1 test within LT 1 was not available, it was only possible to conduct simulation testing by an electrically looping the signal without being able to access OS/OR in LT 1, thus forcing evaluation of the circuit performance within LT 1 to be performed manually.